1. Field of the Invention
The present invention relates to an endoscope comprising, as an image-transmitting member, a multifilament type plastic optical fiber cable in which the section of an image-transmitting portion has a circular or substantially circular shape, which endoscope has a good elasticity can observe even the interior of a narrow blood vessel, and can be effectively utilized as a medical endoscope.
2. Description of the Related Art
With the recent increase of the number of patients suffering from heart diseases, the development of a blood vessel endoscope capable of observing the interior of the coronary arteries is urgently required, and an endoscope comprising, as an image-transmitting member, an optical fiber bundle composed of a great number of bundled glass type optical filaments having a diameter of 10 to 50 .mu. has been developed. Nevertheless, since the glass type optical fiber bundle is rigid and easily broken, it is difficult to pass an endoscope comprising an image-transmitting member composed of glass type optical filaments bundle through the curved and bent coronary arteries, and even if the endoscope can be passed through the coronary arteries, if the filaments are broken this will be life-endangering, and thus the endoscope cannot be practically used. Although a throwaway type blood vessel endoscope is desirable, for example, to avoid infection with AIDS and the like, this desire cannot be satisfied by the glass type optical fiber due to the high cost thereof.
An endoscope comprising an image-transmitting member composed of a multifilament type plastic optical fiber cable also is being developed, and among multifilament type plastic optical fiber cable regarded as being capable of transmitting images, those having a circular section in the image-transmitting portion are disclosed in Japanese Unexamined Patent Publication No. 56-39505 and Japanese Examined Patent Publication No. 59-14570. Nevertheless, in these multifilament type plastic optical fiber cable, as shown in Japanese Examined Patent Publication No. 59-14570, the sections of islands acting as light-transmitting paths in the section of the image-transmitting portion are deformed from the circular shape and have a polygonal shape, and therefore, the light-transmitting characteristics of the light-transmitting paths at the center of the section of the image-transmitting portion of the multifilament type optical fiber cable are substantially different from those of the light-transmitting paths in the peripheral portion of the section of the image-transmitting portion. Accordingly, it is impossible to transmit a sharp image, and such a multifilament type optical fiber cable cannot be utilized as the image-transmitting member of a blood vessel endoscope.
Blood vessel endoscopes having improved performances have been proposed in U.S. Pat. No. 4,872,740, Japanese Unexamined Patent Publication No. 63-197190 and Japanese Unexamined Patent Publication No. 64-908, and in the plastic image-transmitting member used for these endoscopes, light transmitting paths having a substantially circular sectional shape are arranged in a hexagonal staggered form structure in the section of a multifilament type optical fiber cable having an image-transmitting portion having a substantially rectangular peripheral shape, and therefore, the respective light transmitting paths have a substantially circular shape to hexagonal sectional shape. A multifilament plastic optical fiber cable of this type has superior image-transmitting characteristics to those of the conventional multifilament type plastic optical fibers, and it is thought that the plastic optical fiber cable of this type can be used for an endoscope.
Nevertheless, as the result of the investigation made by the present inventors, it was found that multifilament type plastic optical fiber cable as mentioned above have a anisotropy of the flexural regidity and the flexural rigidity in a direction diagonal in the rectangular section of the image-transmission portion that is too large for use as an image-transmitting member of an endoscope, and therefore, it is very difficult to put these plastic optical fibers to practical use.